


Reciprocal Altruism

by kirstenlouise



Category: Sherlock (TV)
Genre: Anal Sex, Dubious Consent, M/M, Oral Sex, Rimming, Sibling Incest, Underage Sex
Language: English
Status: Completed
Published: 2012-12-16
Updated: 2012-12-16
Packaged: 2017-11-21 07:22:36
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings, Underage
Chapters: 1
Words: 2,761
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/595021
Author URL: https://archiveofourown.org/users/kirstenlouise/pseuds/kirstenlouise
Summary: <blockquote class="userstuff">
              <p>Mycroft agrees to do Sherlock's maths homework—for a price. Follow-up to <a href="http://archiveofourown.org/works/373089">Things Truly Wicked</a>.</p>
            </blockquote>





	Reciprocal Altruism

**Author's Note:**

> Many thanks to the lovely ficklepig as well as the anons who gave me some advice on this fic when I was originally trying to bash it out a couple of months ago. I would have left it to rot away on my hard-drive, forever doomed to be unfinished, without them.

"Mycroft, what's the limit of one hundred over x-squared plus five, going to infinity?"

"Zero."

He turned the page of his notes, continuing his transcription as Sherlock resumed his hurried scribbling. It was easy enough to tune out the scratch of his pen, the sound swallowed up into the background as Mycroft converted his shorthand to something more legible.

"What about the limit of seven over x-cubed minus twenty? To infinity."

"It's still—" Mycroft glanced up sharply. "Why are you bombarding me with maths problems?"

"Homework."

Ah, of course. The hated horror of homework. He should have known. Mycroft made a brief note on his cuff in pencil before turning back to his still sulking brother.

"I've no intention of doing your homework."

"Not even—"

"No, Sherlock. Now hush. I've no time for your temper tantrums today. I'm quite busy."

Whatever rejoinder Sherlock had prepared for him came out as a frustrated growl as he flopped back on Mycroft's bed. As was so often the case, Sherlock permitted him nearly two consecutive minutes of silence before groaning melodramatically. Mycroft didn't bother to look away from his notes. Better to deny him the satisfaction.

The groan sounded a second time, louder and, if possible, even more irritating.

"What is it now, Sherlock? Are you having a fit? Shall I send for a medic?"

With a quiet chuckle, he jotted down another cuff note. It wouldn't do to forget his little rendezvous later.

"You could do it for me, you know," Sherlock said.

"I could," he agreed, "but I won't. You're perfectly capable of a little simple calculus."

Mycroft feigned interest in his notes, savoring the huff of frustration Sherlock failed to hold back. Out of the corner of his eye, he caught Sherlock rolling off the bed, bare feet silent on the carpet of Mycroft's room. His hands came down on Mycroft's shoulders a moment later.

"I could make it worth your while." There was no need for Mycroft to turn to know Sherlock was smirking. It was evident from the tone he'd taken. "Are you listening, brother?"

"Intently."

"We both know what you were after last Sunday tea." His voice had no right to be that low or that smooth, Mycroft reflected. Never mind the heat of Sherlock's thin, clever fingers working loose the knot of his tie. " _Punishing_ me."

"Mmm. You'd have let me have it, too, you greedy little slut."

The playful dip of his fingers beneath the collar of Mycroft's shirt was confirmation enough. He nipped at Mycroft's earlobe, more viciously than was perhaps necessary.

"You spoilt me, you know. Dirtied me up. Ruined me, Mycroft."

"Oh, do let's not be dramatic."

Sherlock's hand withdrew, halting its gentle exploration. Mycroft swiveled to face him. 

"You're not the least bit penitent, are you?"

"You wouldn't find me half as fascinating if I were." He favored the boy with a smile, privately delighted by the slight widening of his eyes. He dropped his hand to Sherlock's hip, stroking absently over the sharp lines of his pelvis. "Well? I was under the impression you wanted to exchange."

He pitied the boy, really. He was already all aflush, his pains to hide it beneath a veneer of coquettishness notwithstanding. When Sherlock climbed into his lap, he couldn't help but smile. Sherlock responded in kind, arms twining about Mycroft's neck to steady himself. He bit his lip, prettily. Such an obvious affectation should have seemed garish—entirely too artificial to be at all appealing—but it was a pleasing affectation all the same.

Mycroft slid the pad of his thumb over Sherlock's mouth, catching on one of his sharp little teeth.

"Go on," he encouraged. "Suck."

Sherlock's face flooded with red for the briefest second before the pink tip of his tongue flicked out, wet and tentative against Mycroft's thumb. He pushed it a little deeper, pleased at the utter lack of resistance as Sherlock's mouth closed softly around the digit. Suction followed moments later, accompanied by soft, suckling noises that sent his cock swelling.

He slipped his thumb free, gently brushing away the lingering saliva along Sherlock's bottom lip. 

Sherlock batted at his hand. "I can do it myself, Mycroft."

"I've no doubt." He leaned back in his chair and laced his hands over his abdomen. "I think I'll have you naked now. Eyes on me."

There were few sights more pleasing than the tremulous striptease of a boy forced to watch you watching him undress. Sherlock performed with stiff-necked pride, beginning with the buttons of his shirt until he stood bare in front of Mycroft, naked as the day he was born and infinitely more intriguing. He wasn't surprised to find Sherlock hard, not at this stage. It was the thrill of obedience, coupled with the possibility of insubordination; the mild perversion flavored with a deeper taboo.

Sherlock liked this, even if he was too proud to admit it.

"Closer now, pet."

Sherlock held back, wary. "What for?"

"Don't worry. You'll enjoy yourself, I'm sure."

He caught Sherlock by his soft, slim hips and tugged him forward. It was almost too easy to flip him round. Sherlock bent over all on his own, arse presented in vulgar delight. Such a smooth, plump bottom. Sherlock responded with a sharp intake of breath as Mycroft's hands spread his cheeks wide.

He blew lightly over the exposed hole, eliciting a delicious, full-bodied quiver. The barest touch of his lips made Sherlock jerk away.

" _Mycroft!_ "

"How did you imagine I'd get you wet enough to take my cock, you silly boy?"

Sherlock sunk back to his former position, aided by Mycroft's hand planted firmly on the small of his back. Mycroft dove back in immediately, trailing wet little kisses over his brother's pink little hole and running his hands up and down the backs of his long, slender thighs. The way Sherlock was responding, he wouldn't even need his fingers. The flicker of Mycroft's tongue along his cleft and the light nosing over his perineum were making him unfold without difficulty.

He spat and Sherlock reared up. "That's revolting, Mycroft."

Mycroft ignored his outburst and pressed him back to the desk. The gob of saliva was already slicking its way down the curve of Sherlock's arse. Mycroft spread it with his tongue, licking and laving, harder now. Sherlock wriggled atop his desk, breathing hard and shaking with every pass of Mycroft's tongue over his wet little hole. He was so lovely like this. It was a wonder he hadn't considered it before.

Mycroft used both thumbs to spread his pucker wider. It gave easily under the pressure of his fingers, exposing a soft, pink centre. He let just the tip of his tongue slip inside, fighting the urge to smile when Sherlock gave a jerky press back toward his face. It was almost too easy to sink his thumbs into Sherlock, both at once.

"You're ready for me now, aren't you?" he remarked, pleased by the ripple down Sherlock's spine as he fought not to roll his arse into Mycroft's hands. "Ready to sink back onto my prick. Admit it."

Sherlock's voice was defiant, if breathy. "I hardly need to _encourage_ you."

"Admit it," Mycroft clarified, "or I'll leave you naked and wanting and you'll have no one but yourself to blame."

There was something greedy about the way the boy immediately pressed back, trying to take Mycroft's fingers up past the knuckle. The reply followed, grudgingly. "Just give me your prick, Mycroft."

"Is that how you ask nicely?" He gave a final twist of his thumbs before withdrawing. Sherlock gave a choked moan. "Don't be insolent. It isn't becoming."

"And having your fingers jammed up my arse is?"

Mycroft gave him a good smack. "On the bed, little brother."

On shaky legs, Sherlock did as he was bidden. He sat cross-legged on the bed, his hard little cock jutting up from his lap. Mycroft watched with interest as he started to play with it with little twists of his hand. His belly was already sticky—he'd come early on, then.

"I want to see it this time, Mycroft."

"See what, precisely?"

Sherlock glared, pairing the expression with a sharp tug of his cock. "Your _prick_ , you berk. Last time you only let me feel it."

Mycroft remembered the last time quite well. As well as he remembered the state of his trousers after. Better to lay them aside this time. He undressed without fanfare, quickly and efficiently. He had no interest in putting on a show. He stood at the edge of the bed, letting Sherlock look at him and watching the boy's Adam's apple bob as he studied Mycroft's prick, half-hard and heavy along his thigh.

A hand reached out to settle on his belly. Mycroft covered it with his own and guided it gently between his legs, letting Sherlock's fingers curl into the wiry hair there before wrapping them around his cock. He let his free hand curl around the base of Sherlock's neck, pulling him forward a bit.

"Open your mouth."

"You didn't say I was supposed to suck you off." Sherlock's eyes darted off to the side. Mycroft bumped his cock against his cheek, smearing precome along the sharp ridge of a cheekbone and bringing a fresh flush to the surface of his skin. "Knock it off, Mycroft."

"You can suck me now or you can suck me after I've come in you."

"You're revolting."

"We've all of us our flaws. Now be a good boy and open wide."

Sherlock's eyes flashed defiance, but his lips parted enough for Mycroft to feed him the head of his cock. He didn't need to take more than that. Not today. Sherlock sucked with his eyes off somewhere to the side. Mycroft let him. He'd come round, sooner or later. When he was hard he pulled back with a soft pop, taking a moment to admire the wet shine on Sherlock's mouth and chin before pressing him back on the bed and spreading his knees.

Sherlock cried out at the shove of his cock, sharp and high enough to shatter crystal. He grit his teeth and glowered. "That _hurt_ , Mycroft."

"And now it's over," he soothed, running a hand along the boy's cheek. "You should thank me for not dragging it out."

His little cock had deflated in the interim, but no matter. Boys always went soft the first time. Mycroft would soon have him stiff again. Sherlock gave another yelp as he began to move, a slow glide in and out of his snug little passage. He bent to kiss away Sherlock's scowl, lapping at the seam of his mouth.

"I'll make you come if you're a very good boy. Would you like that?"

His mouth quivered against Mycroft's, his hips rising ever so slightly. That was answer enough. Mycroft hooked an arm under Sherlock's back and rolled them over without breaking contact, folding Sherlock down against his chest. His back was beading with sweat as Mycroft stroked over the sharp knobs of his spine, all the way down to where they were joined.

Sherlock was stuffed to bursting, the poor thing. Mycroft massaged his taut little hole, trying to relax him as he lay sniffling against Mycroft's shoulder.

Mycroft sucked two fingers into his mouth and wet them liberally before reaching back down to where he was pressed inside Sherlock. He rubbed lightly around the rim as Sherlock rocked back and forth in an attempt to work himself loose. He let a groan of pleasure escape his lips, delighting in the way Sherlock flushed at his praise. Sherlock kept shifting, his muscles squeezing Mycroft deliciously as he tried to get comfortable on Mycroft's prick.

Sherlock rubbed his face into Mycroft's shoulder, mumbling, "Why haven't you come yet?"

"Don't tell me you're bored, little brother."

Sherlock sat back on him with a scowl. "You're just _lying_ there."

"I don't recall complaining when you were too frightened to even shift your knees apart for me," Mycroft responded mildly. He stroked along Sherlock's belly, sticky under his fingers. "Are you so horrified by the thought of having to work for what you want?" Without waiting for an answer, he cupped Sherlock's cock in his hand. Sherlock's hips twitched forward. " _Quid pro quo_ , brother."

For the longest time, Sherlock only stared, gaze calculating. At the last, he gave a minor rock of his hips atop Mycroft, as if testing the waters. Mycroft rubbed his prick encouragingly. He was firming up again quickly. Typical, for a boy his age. Mycroft wouldn't begrudge him a second climax or even a third, if he behaved. He liked the sound of that—Sherlock gasping, bucking into his fist, oversensitized and half-raw from rubbing his little cock against the wood grain of Mycroft's desk and still unable to help himself from rutting against him.

Sherlock's palms dug into Mycroft's shoulders as he raised himself a few inches before sinking back down, brow furrowed. His knees rubbed sharply against Mycroft's sides, his nails bitten and uneven, his movements ungainly and unpracticed. Perfect. Mycroft drank in the sight of him with his flushed skin and his slim cock bobbing against his belly. He’d imagined this before, a hundred times in between now and the afternoon he’d rubbed himself off between Sherlock’s thighs, but the reality of it was quite different. More thrilling than he’d imagined, breaking one of the few taboos left to them. He typically preferred his partners more skilled and better versed in the art of lovemaking, but it was intriguing to be the teacher for a change—to lie back and simply luxuriate in the sight and the sensation as Sherlock rode his cock.

"You're enjoying this, aren't you? Despite your best efforts."

"Piss off," Sherlock muttered. His breathlessness dulled the impact somewhat. As did the fact that Mycroft's jibe hadn't made him slow down a bit. If anything, his movements were growing increasingly frantic, making it clear that he wasn’t simply _tolerating_ Mycroft, but actively appreciating the sensation created by his cock nestled snugly inside him. "You said you'd let me come."

"Am I hindering you in some way?"

"What happened to _quid pro quo_?"

"Nothing at all. Need I remind you that this was your idea?" 

Sherlock grasped his own cock and gave it a sharp tug. Mycroft rewarded his initiative by gently rolling Sherlock's testicles in his palm. He made a choking sound, breath catching in his throat. It was incredible, to think that something so simple could be so utterly gratifying. Sherlock's muscles seized a final time before he splattered hot ropes of semen over Mycroft’s chest, hatred be damned. 

"I hate you," he said. It stung not at all. There was little force behind it. At this point, his distaste was merely routine. Expected, but not backed by any sort of enthusiasm.

Mycroft smiled gently. "Yes, I'm quite certain that you do. On your hands and knees now, if you would."

He took a moment to admire how red Sherlock was, his hole stretched and glistening, before sliding back inside. Sherlock let out a muffled cry as Mycroft began to thrust rapidly in and out, the slap of flesh on flesh obscenely loud. He was exquisite even from behind. Perhaps even more so, what with his delicate, bird-boned shoulders and the pronounced notches of his spine. Mycroft pressed a kiss between his shoulder blades, pinched into a high peak.

"If I'd known setting you on my prick would make you so pliant, I'd have done it much sooner. Thinking of all the punishment I've meted out, ineffectively..." He grunted. "When all along, it was this easy? It's simply horrifying." His orgasm hit only a moment later, a torrent of heat and ecstasy. He collapsed onto Sherlock's back with a sigh, cock still buried inside him. "Have I mentioned how extraordinarily lovely I find you like this?"

Sherlock grumbled underneath him. "I can feel every biscuit you've ever stuffed your face with."

"Don't ruin the moment."

"You're absolutely the most revolting human being I know. If you are human."

"Regrettably so," Mycroft sighed. He closed his eyes. "It would take a far better man than I to resist your particular charms, rudimentary and common though they may be. A far better man than any man I've ever met, perhaps. Would that I were more than a simple, earthly—"

"Pervert," Sherlock finished. "Deviant, if you prefer, but most definitely some breed of degenerate."

Mycroft smiled against the back of his neck. "Yes, I love you, too."


End file.
